1. Field of the Invention
The present invention relates to a liquid crystal display used in a display section of an electronic apparatus and a substrate for a liquid crystal display used in the same.
2. Description of the Related Art
A liquid crystal display has a pair of substrates having transparent electrodes on surfaces thereof opposite to each other and a liquid crystal layer sealed between the substrates. In the liquid crystal display, a voltage is applied between the transparent electrodes to drive the liquid crystal, thereby controlling the transmittance of light at each pixel. Recently, there are increasing demands for liquid crystal displays, and requirements for liquid crystal displays are also diversifying. In particular, improvement of display quality is strongly demanded.
Active matrix liquid crystal displays that are the main stream of liquid crystal displays have a thin film transistor (TFT) as a switching element at each pixel. The thickness (cell gap) of the liquid crystal layer of a liquid crystal display is maintained by spherical spacers or rod-shaped spacers. The spherical spacers or rod-shaped spacers are made of plastic or glass. Normally, those spacers are dispersed on one of the substrates at a spacer dispersing step. The two substrates are thereafter combined and are pressed from the outside such that a cell gap on the order of the diameter of the spacers is maintained.
However, since the spacers are provided also in the pixels, they can result in an alignment defect of the liquid crystal and leakage of light. When there is an alignment defect of the liquid crystal or leakage of light, a reduction of contrast or glare on the display screen occur to reduce display quality. Increases in the size of substrates have made it difficult to disperse spacers uniformly. When spacers are ununiformly dispersed, variations of the cell gap occur in the plane of the substrate, which results in irregularities of luminance. In particular, liquid crystal displays of the IPS (in-plane switching) mode and MVA (multi-domain vertical alignment) mode undergo greater changes in luminance in response to changes in the cell gap compared to TN (twisted nematic) mode liquid crystal displays. Therefore, control must be performed to provide a more uniform cell gap in order to achieve display without irregularity of luminance. Further, since the trend towards finer pixels has resulted in reductions in the area of one pixel, the area occupied by a spacer relative to a pixel is becoming greater, and the influence of spacers on display quality is therefore becoming more significant.
The above-described problem is solved by the use of pillar spacers (post spacers) constituted by a photosensitive resin and formed at a photolithographic step. Since pillar spacers are formed at a photolithographic step, they can be provided in an arbitrary density in regions which are shielded from light with a light-shield film (herein after also referred to as black matrix (BM)). Since this prevents the occurrence of alignment defects of a liquid crystal or leakage of light at pixels, neither reduction of contrast nor glare occurs. Since pillar spacers allow a film to be formed with a uniform thickness (height), a uniform cell gap can be maintained throughout the plane of a substrate. Therefore, there will be no irregularity of luminance attributable to variation of a cell gap. Thus, the use of pillar spacers makes it possible to provide a liquid crystal display having high display characteristics.
FIG. 7 shows a configuration of an opposite substrate of a liquid crystal display according to the related art. FIG. 8 shows a sectional configuration of the liquid crystal display taken along the line X—X in FIG. 7. As shown in FIG. 7 and FIG. 8, a BM 110 in the form of a grid for shielding light is formed on a glass substrate 107 that constitutes an opposite substrate 104. Although not shown, TFTs, gate bus lines and drain bus lines are formed on a TFT substrate 102 in regions which are shielded from light by the BM 110. Pixel regions of the opposite substrate 104 are defined by the BM 110. Since the BM 110 also shields storage capacitor bus lines (not shown) formed on the TFT substrate 102 across the pixel regions, two apertures α and β indicated by broken lines in FIG. 7 constitute one pixel.
A color filter (CF) layer in any of red (R), green (G) and blue (B) is formed at each of the pixel regions of the opposite substrate 104. For example, CF layers R, G and B are formed like stripes extending in the vertical direction in FIG. 7. A common electrode 116 constituted by a transparent conductive film is formed throughout the substrate over the CF layers R, G and B. An alignment film 115 is formed throughout the substrate over the common electrode 116.
The TFT substrate 102 has a pixel electrode 112 formed in each of the pixel regions on the glass substrate 106. An alignment film 114 is formed throughout the substrate over the pixel electrodes 112.
A liquid crystal 108 is sealed between the opposite substrate 104 and the TFT substrate 102. A cell gap is maintained by pillar spacers 118 which are formed in the regions shielded from light by the BM 110 on the opposite substrate 104. In FIG. 7, the pillar spacers 118 are formed in regions which are located on the BM 110 shielding the storage capacitor bus lines from light and in which the CF layers B are formed, one spacer being provided for every six pixels.
A description will now be made with reference to FIGS. 9A to 9D on a method of manufacturing an opposite substrate 104 and a liquid crystal display having the same according to the related art. First, as shown in FIG. 9A, a film of a metal such as chromium (Cr) or a black resin film is formed throughout a top surface of a transparent and insulating glass substrate 107 and is patterned to form a BM 110. Next, as shown in FIG. 9B, CF layers in R, G and B are sequentially formed like stripes using a pigment-dispersion type photosensitive colored resins, or the like. Next, as shown in FIG. 9C, a transparent conductive film such as an ITO (indium tin oxide) is formed throughout the substrate over the CF layers R, G and B using a sputtering process to form a common electrode 116. Prior to the formation of the common electrode 116, the surface of the substrate may be planarized by applying an acrylic resin or epoxy resin or the like on the CF layers R, G and B to form an overcoat layer.
For example, an acrylic resin type negative photosensitive resist is then applied throughout the substrate. Subsequently, as shown in FIG. 9D, pillar spacers 118 are formed in an arbitrary density in arbitrary positions using a photolithographic process. The pillar spacers 118 are provided only on the BM 110. Since the accuracy of the height of the pillar spacers 118 is important, they are preferably formed on CF layers in any one of the colors R, G and B if possible considering differences in thickness between the CF layers R, G and B. An opposite substrate 104 is completed through the above-described steps.
Next, an alignment film 115 is applied throughout a top surface of the opposite substrate 104, and an alignment film 114 is applied through a top surface of a TFT substrate 102 which has been manufactured through an array fabrication step. Next, the alignment films 114 and 115 are rubbed in a predetermined direction. The substrates 102 and 104 are then combined such that their surfaces on which the alignment films 114 and 115 are formed face each other, and a liquid crystal is injected between the substrates 102 and 104. A liquid crystal display is manufactured through the above-described steps. The pillar spacers 118 may be formed on the TFT substrate 102.
Patent Document 1: Japanese Patent Application Laid-Open No. JP-A-2000-305086
Patent Document 2: Japanese Patent Application Laid-Open No. JP-A-2001-75500
Patent Document 3: Japanese Patent Application Laid-Open No. JP-A-2001-201750
Normally, the pillar spacers 118 are formed with a dimension in the range of 10˜30 μm×10˜30 μm square and a height in the range from 4 to 5 μm. At the rubbing step, the alignment film 114 or 115 is not sufficiently rubbed in regions around the pillar spacers 118. Therefore, the alignment film 114 or 115 in the regions cannot be provided with a predetermined alignment regulating force, and regions having liquid crystal alignment defects are formed around the pillar spacers 118. The regions having liquid crystal alignment defects are also formed around the pillar spacers 118 because of the influence of the spacers themselves. It is therefore necessary to provide the pillar spacers 118 in positions where their neighborhoods can be sufficiently shielded from light with the BM 110 in order to prevent any reduction in display quality even when there are irregularities in the alignment of the liquid crystal.
Recent demands for liquid crystal displays with higher definition and higher transmittance have resulted in a trend toward an improved aperture ratio which is to be achieved by forming the BM 110 with a smaller width here. A problem therefore arises in that it is difficult to reserve sufficient spaces to shield the neighborhoods of the pillar spacers 118 from light. In some occasions, however, the BM 110 must be formed with a great width to shield the neighborhoods of the pillar spacers 118. As a result, the aperture ratio of pixels is decreased, which results in the problem of a reduction in the display luminance of a liquid crystal display.
Physical properties of the material of which the pillar spacers 118 are formed, e.g., compressive displacement and plastic deformation are important in designing the density in which the pillar spacers 118 are to be provided. Therefore, a design is employed which provides the spacers with flexibility to follow up thermal expansion and contraction of the liquid crystal and hardness to endure the application of a pressure. Pillar spacers 118 designed to satisfy such conditions are normally provided in a density of about one spacer per several pixels. When the BM 110 is formed with a great width only in positions where the pillar spacers 118 are provided or when no particular light shielding is performed in the neighborhoods of the pillar spacers 118 conversely, a problem arises in that the transmittance of pixels in the neighborhoods of the pillar spacers 118 becomes lower than that of other pixels and therefore results in display irregularities that are visually perceived on the screen.